1. Technical Field
The invention is directed to a process for device fabrication in which an energy sensitive resist material is used.
2. Art Background
Devices such as integrated circuits are complex structures made of a variety of materials. These materials are precisely configured to form the desired device by a variety of processes. A lithographic process is frequently used to transfer the desired configuration into a substrate to fabricate such devices.
Lithographic processes use intermediate materials frequently referred to as resists. A positive or negative image of the desired configuration is first introduced into the resist by exposing it to patterned radiation which induces a chemical change in the exposed portions of the resist. This chemical change is then exploited to develop a pattern in the resist, which is then transferred into the substrate underlying the resist.
The efficacy of a lithographic process depends at least in part on the resist used to transfer the pattern into the substrate. Certain types of resists offer particular advantages in the context of specific lithographic processes. For example, solution-developed resists are designed to have absorption characteristics appropriate for use at certain exposure wavelengths. It is axiomatic that, if the resist material is opaque to the exposing radiation, the exposing radiation will not be transmitted into the resist material and the desired chemical change will not occur. Therefore it is important to select a resist material that has the appropriate light transmission characteristics at the wavelength of the exposing radiation. Other considerations that drive the selection of an appropriate resist material include the etch resistance of the resist after it has been exposed and developed.
A variety of resist materials are employed in lithographic processes for device fabrication. One class of resist materials contains a polymer which has certain functional groups (e.g. alcohol (OH); phenol (C.sub.6 H.sub.5 OH); carboxylic acid (COOH); etc.). A certain portion of these functional groups are "masked," i.e., the hydrogen atom is removed and replaced by moieties referred to as protecting groups. These protecting groups are removable from the polymer by acidolysis and/or hydrolysis. A polymer with a significant number of these protecting groups has a very different solubility in developer solutions (typically aqueous base solutions) than a polymer with substantially fewer of these protecting groups. Examples of protecting groups include acetals, ketals, bis(trimethylsilylmethyl)methyloxy, t-butoxycarbonyloxy, t-butyl esters, and t-butyl ethers which are cleavable from the functional groups by acidolysis or hydrolysis. The functional groups from which the protecting groups have been cleaved are referred to as unmasked functional groups.
The resist materials also contain an energy-sensitive material in combination with the polymer. When exposed to a certain energy (energy of a particular wavelength (e.g. 248 nm) or type (electron beam)) a moiety is generated from the energy-sensitive material which effects the cleavage of the protecting group, thereby "unmasking" the functional group. When the protecting group is an acid labile group, i.e. it is removed in the presence of acid, the energy sensitive material is typically a photoacid generator (PAG). The greater the number of protecting groups that are cleaved from the polymer, the greater the chemical contrast between the polymer exposed to radiation and the polymer not exposed to radiation. This chemical contrast between the unexposed resist material and the exposed resist material is exploited to develop a pattern in the resist material.
One problem associated with the above-described materials is that some of the byproducts of the energy-induced reactions outgas from the resist material. Resist outgassing during exposure is a problem in deep UV lithography (248 nm, 193 nm, 157 nm etc.), extreme UV lithography, ion beam lithography and e-beam lithography (e.g. direct write and SCALPEL.RTM. (scattering with angular limitation projection electron beam lithography)). In UV lithography the outgassed constituents potentially condense on and damage the optics (e.g. the lenses) in the exposure tool. Outgassed aromatic compounds are especially deleterious in processes in which the exposure wavelength is 193 nm and less because the aromatic compounds absorb light at these exposure wavlengths. Consequently, outgassed aromatics that condense on the optics in the exposure tool reduce the efficiency of the tool by absorbing some of the radiation transmitted through the optics. Also, since the concentration of these absorbing species that condense on the optics tends to be non-uniform, the uniformity of the exposure is also adversely affected. In electron and ion beam lithography, the outgassed constituents potentially interfere with the high vacuum environment needed for exposure in addition to degrading the lithographic tool performance.
One proposed solution to the problem of resist outgassing is the use of a nitrogen purge to keep the outgassed constituents from condensing on the critical optical elements. However, this solution does not limit the amount of outgassing that occurs, and may not be feasible for all optical elements or lithographic tools. Furthermore, the adverse effects of outgassed materials that have a low vapor pressure are not ameliorated by a nitrogen purge. Accordingly, resist materials and lithographic processes that reduce or eliminate the problems associated with resist outgassing are sought.